C HAPTER 3 - S TOICHIOMETRY

3.1 – A TOMIC M ASSES Carbon-12, the relative standard C-12 is assigned a mass of exactly 12 atomic mass units (amu) Masses of all elements are determined in comparison to the 12 C atom which is the most common isotope of carbon Comparisons are made using a mass spectrometer Atomic Mass (average atomic mass, atomic weight) Atomic masses are the weighted average of the naturally occurring isotopes of an element Atomic mass does not represent the mass of any actual atom Atomic mass can be used to “weigh out” large numbers of atoms

3.2 – T HE M OLE Avogadro’s number x units = 1 mole Measuring moles An elements atomic mass expressed in grams contains 1 mole of that element grams of carbon = 1 mole of carbon grams of carbon = x atoms of carbon 1 mole of carbon = x atoms of carbon

3.3 – M OLAR MASS Molar mass is the mass in grams of one mole of a compound The sum of the masses of the component atoms in a compound is calculated using the PTE What is the molar mass of ethane (C 2 H 6 )? 2 moles of C = 2 x g = g 6 moles of H = 6 x g = g g/mole ethane

3.4 – P ERCENT C OMPOSITION A percentage is “The part, divided by the whole, multiplied by 100” Percent Composition Calculate the percent composition of each element in the total mass of a compound (or sample) (# atoms of the element)(atomic mass of element) x 100 (molar mass of the compound) What is the % of sodium in sodium chloride? NaCl has a molar mass of 58.5 g/mol (23 /58.5) x 100 = 39%

3.5 – D ETERMINING THE FORMULA OF A COMPOUND Empirical formula – simplest whole number ratio of elements in a compound Determine the percentage of each element in your compound Treat % as grams and convert grams to moles using the mass from the PTE Find the smallest whole number ratio of atoms (multiply by an integer to make them whole numbers). A compound contains 63.5% Silver, 8.2% Nitrogen and 28.2% Oxygen. What is the empirical formula for this compound? Ag = 63.5 g/ 108 g =.59 mol N = 8.2 g / 14 g =.59 mol O = 28.2 g / 16 g = 1.76 mol This is a 1:1:3 ratio Formula for this compound is AgNO 3

3.5 – D ETERMINING THE FORMULA OF A COMPOUND Molecular formula – actual ratio of elements in a compound Calculate the empirical formula mass Divide the known molecular mass by the empirical formula mass deriving a whole number, n Multiply the empirical formula by n to derive the molecular formula Ethane gas has an empirical formula of CH 3 and a molecular mass of 30 g/mol. What is the molecular formula for ethane? Empirical formula mass is = 15 g/mol 30/15 = 2 2(CH 3 ) = C 2 H 6

3.6 – CHEMICAL EQUATIONS Chemical Reactions Reactants are listed on the left had side of the arrow Products are listed on the right hand side of the arrow Atoms are neither created or destroyed All atoms present in the reactants must be accounted for among the products, in the same number No new atoms may appear in the products that were not present in the reactants REACTANTS yield PRODUCTS Zn (s) + 2 HCl (aq)  ZnCl 2 (aq) + H 2 (g)

3.6 – CHEMICAL EQUATIONS The meaning of a chemical reaction Solid - (s) Liquid - (l) Gas - (g) Dissolved in water (aqueous solution) - (aq) Relative numbers of reactants and products Coefficients give atomic/molecular/mole ratios Zn (s) + 2 HCl (aq)  ZnCl 2 (aq) + H 2 (g)

3.7 – B ALANCING C HEMICAL E QUATIONS Determine what reaction is occuring It is sometimes helpful to write in word form Write the unbalanced equation Focus on writing correct atomic and compound formulas Balance the equation using an atom inventory It is often helpful to work systematically from left to right Include phase information Hydrogen and oxygen gases combine to form liquid water

3.8 – S TOICHIOMETRIC C ALCULATIONS Balance the chemical equation Convert grams of reactant (or product) to moles, if required Compare moles of the known value to moles of desired substance (mole ratio) Convert from moles back to grams if required If 5 g of Zn reacts with excess hydrochloric acid, how many grams of zinc chloride are formed?

3.9 – C ALCULATIONS INVOLVING A L IMITING R EACTANT A concept of limiting reactant The limiting reactant controls the amount of product that can form Solving limiting reactant problems Convert grams of both reactants to moles Use mole ratios to determine which reactant forms the least amount (moles) of desired product Convert the least moles to grams (if required) If 5 g of Zn reacts with 5 g of hydrochloric acid, how many moles of hydrogen gas are formed?

3.9 – C ALCULATIONS INVOLVING A L IMITING R EACTANT Calculating percent yield Actual yield – what you got by actually doing the experiment Theoretical yield – what stoichiometric calculations say the reaction should have produced. Actual yield (data table) x 100 = % yield Theoretical yield (Stoich) 5 g of Zn reacts with excess hydrochloric acid. 9.4 g of zinc chloride are formed. What is the percent yield for this reaction?